A computer system may produce a duplicate of a storage volume having stored original data when a backup copy of data to be stored in a storage device is created. In such a case, the storage volume having stored the original data and the storage volume having stored the duplicate data must have the same contents and keep consistency. For that, it is necessary to suspend the update of the storage volume having stored the original data until the duplicate is completely produced. But, the computer system or the like required to have high availability might not able to stop access to the storage volume having stored the original data.
Therefore, there has been developed a technology called a snapshot by which, even when consistency between the storage volume having stored the original data and the storage volume having stored the duplicate data at a prescribed point in time is held and then the original data is updated after a prescribed point in time, the original data at a prescribed point in time can be referred.
According to the snapshot technology, when the original data is updated after the prescribed point in time when the consistency must be kept, the data at the prescribed point in time when the consistency must be maintained is stored in another storage volume. Specifically, the original data remains as it is when it is not updated, and when it is to be updated, the data at the prescribed point in time when the consistency must be maintained is stored in the latter storage volume.
However, it is general that the storage capacity which is provided as the second storage volume as described above is smaller than that of the storage volume storing the original data in order to cut costs, or the like.
Therefore, according to the conventional snapshot control, the period in which the consistency between the storage volume storing the original data and the latter storage volume storing the duplicate data depends on an updating frequency of the original data and the storage capacity of the latter storage volume.